Legal decisions often require logical reasoning about the mental states of people who perform gruesome behaviors. We use functional magnetic resonance imaging (fMRI) to examine how brain regions implicated in logical reasoning are modulated by emotion and social cognition during legal decision-making. Participants read vignettes describing crimes that elicit strong or weak disgust matched on punishment severity using the US Federal Sentencing Guidelines. An extraneous sentence at the end of each vignette described the perpetrator’s personality using traits or biological language, mimicking the increased use of scientific evidence presented in courts. Behavioral results indicate that crimes weak in disgust receive significantly less punishment than the guidelines recommend. Neuroimaging results indicate that brain regions active during logical reasoning respond less to crimes weak in disgust and biological descriptions of personality, demonstrating the impact of emotion and social cognition on logical reasoning mechanisms necessary for legal decision-making.